The biggest issue with prot pally in raids cant speak much for pvp is the dedication and requirement ceiling is much higher specially when compared to a warrior. You can be an average to subpar warrior and still be a decent tank called upon where as with prot pally you kinda need to know your stuff specially at end game raiding. The lack of any ability to snap agro hurts a bit as a pally you need to a bit precise with your rotation as all I takes is a good oopsy for things to go south for you as you don’t have really any strong abilities to snap agro back. Any tank swap bosses you are at a complete disadvantage and can screw things up very easily if you are not a meticulous player.
You want to prot pally in end game you have to have some form of dedication to your player and playstyle your gonna need to know the right gear you will need and rotation as you have very small room for error. Its a balancing act between mana management and making sure your just squishy enough to get smashed in the face for your procs but not to squishy that your healers arent hating life when you do. Prot pally can and have been used as end game tank both as MT and OT however the MT is saved for those who can really be at the top of their game as a pally because it is too easy to have a oopsy that causes everything to go south very quickly.